Decrease in GABA immunoreactivity and alteration of GABA metabolism after kindling in the rat hippocampus

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Abstract
The kindling model of epilepsy, induced by tetanic stimulation of Schaffer collateral/commissural fibers, was studied in the rat hippocampus. Gamma-aminobutyric acid immunoreactivity was used to quantify the number of GABA-immunoreactive somata per mm2 in CA1 region, 28 days after the last generalized seizure. Comparison of the numbers obtained from kindled animals with those from controls, showed a significant decrease (18%) on the ipsilateral stimulated side but none on the contralateral side. In control rats injection of the GABA-transaminase inhibitor, amino oxyacetic acid (AOAA), led to a 46% increase in the number of cell somata immunoreactive for GABA. This probably results from an accumulation of GABA, reflecting GABA synthesis by glutamate decarboxylase (GAD) activity, in somata of interneurons that had initially a GABA content below the immunocytochemical detection threshold. In kindled rats, 31 days after the last seizure, the number of GABA-immunoreactive cells that could be observed after AOAA-treatment was significantly lower (35% ipsilateral and 25% contralateral) when compared to AOAA-treated controls. This suggests that in kindled animals a GAD dependent increase in GABA content did not take place in a subpopulation of interneurons. The observations for kindled rats are interpreted as a long-term decrease in GABA content and as an alteration in GABA turnover in a subpopulation of interneuron somata, the latter possibly due to a decrease in GAD activity. The long-term enhanced seizure sensitivity, characteristic for kindled animals, may be due to a decreased GABAergic inhibitory control of the neuronal circuitry in the CA1 region of the hippocampus.